A first wall in a nuclear fusion device refers to equipment in general which directly face plasma generated in a reactor, specifically including a divertor, a blanket surface, a limiter, or the like. As the first wall directly faces plasma, it receives severe heat and particle load from high temperature plasma. Therefore, the first wall is required to maintain its structural integrity and serve as a shielding body against plasma for the surrounding structure without causing negative effects to the plasma.
Therefore, the first wall is required to have a function of enduring a high heat load due to high temperature plasma and removing the heat. In order to achieve such a heat removing function under the high heat load, it is necessary to configure the first wall by a material with high heat conductivity.
FIG. 5 illustrates an example of a conventional heat receiving tile formed of carbon fiber composite material used for a divertor of a nuclear fusion device, and in particular, illustrates a heat receiving tile suitable for a divertor of a Tokamak type nuclear fusion device.
The divertor of the Tokamak type nuclear fusion device receives the highest heat load in the nuclear fusion device since a kinetic energy of charged particles injected to the divertor is provided thereto as heat. Therefore, the divertor is required to have a function of enduring such a high heat load and removing the heat.
A heat receiving tile 50 of the divertor in FIG. 5 is provided with a heat receiving block 51, which is formed of a material having less negative influence on plasma, on its surface facing the plasma in order to protect a cooling structure from a sputtering due to ion radiation or a heat impact caused by plasma disruption.
The heat receiving block 51 is desired to be formed of a material having a small atomic number which has less negative influence on the plasma, since particles are emitted from the surface of the heat receiving block 51 and mixed in the plasma due to the sputtering or the like, causing decrease in plasma temperature and degradation of confinement performance. A carbon fiber composite material (CFC) is a typical example of such a material, considering heat conductivity.
In the nuclear fusion device which performs a long-term discharge, a surface temperature of a divertor exceeds a service limit temperature of component materials, going beyond heat capacity of the materials configuring the divertor. Accordingly, a through hole 52 is formed at the center of the heat receiving block 51 and a cooling pipe 53 is inserted therethrough. A cooling pipe formed of a copper alloy which has a high heat conductivity and a high strength such as chromium-zirconium copper (CuCrZr), for example is employed as the cooling pipe 53.
Upon operation of the nuclear fusion device, a cooling material such as water is passed though the inside of the cooling pipe 53 so as to forcibly remove the heat that the heat receiving block 51 received. This prevents the heat receiving block 51 from being damaged exceeding its service limit temperature.
However, there are not good in joinability between the heat receiving block 51 formed of carbon material such as CFC and the cooling pipe 52 formed of a copper alloy such as CuCrZr, and also there is a great difference in the coefficient of thermal expansion.
Accordingly, in order to efficiently conduct a heat energy received from the plasma to the cooling pipe 53 and also to absorb the difference in the coefficient of thermal expansion, a buffer material 54 formed of a cupper material such as CuW is interposed between the heat receiving block 51 and the cooling pipe 53.
The heat receiving block 51 and the buffer material 54, and the buffer material 54 and the cooling pipe 53 are metallurgically joined by brazing using a Cu—Mg based or Ti—Cu based joining material with high heat conductivity so as to reduce the resistance of heat transfer as much as possible. Namely, there is a first brazing portion 55 between the heat receiving block 51 and the buffer material 54, and there is a second brazing portion 56 between the buffer material 54 and the cooling pipe 53.
Note that a fitting groove 58, through which a rail for fixing the heat receiving tile 50 to other equipment is passed, is formed on a surface of the heat receiving block 51 opposite to a heat receiving surface 57.
Note that, the heat receiving block 51, the cooling pipe 53, and the buffer material 54 significantly differ in coefficient of thermal expansion from each other as follows: 1×10−6 for the heat receiving block 51; 2×10−5 for the cooling pipe 53; and 1×10−5 for the buffer material 54.
Therefore, when the materials shrink in a high temperature processing during the brazing process, especially in a temperature dropping, the heat receiving block 51 and the buffer material 54 with small thermal expansion coefficient cannot follow the shrinkage of the cooling pipe 53 with large thermal expansion coefficient, and therefore defects tend to occur at joining parts between them.
As a result, a crack 59 sometimes occurs in the heat receiving block 51 near the bonding surface of the buffer material 54, and a peeling 60 sometimes occurs between the heat receiving block 51 and the buffer material 54. The occurrence of this crack 59 and peeling 60 results in the heat conductivity declining and in the cooling efficiency of the heat receiving block 51 declining.
Patent Literature 1 describes a high heat resistant structure component that a graphite portion and a metal portion are coupled to each other via a brazing layer and an intermediate layer is disposed between the metal portion and the brazing layer. Patent Literature 1 describes that this special intermediate layer absorbs the difference in thermal expansion coefficient between the different materials, enabling a solid coupling between the graphite and the metal.
The high heat resistant structure component of Patent Literature 1 could endure a heat cycle load applied during operation of the nuclear fusion device and prevent an extreme deformation and a material crack of the structure component.
However, as brazing is performed in a temperature of 850° C. to 1900° C., the component sometimes cannot endure a high temperature processing applied during a manufacturing process of the component, and therefore a yield of the product is not considered to be high.